This invention relates to the generation of seismic energy in a medium by the use of seismic sources of the type called air guns, and more particularly, to a method and apparatus for monitoring the firing of such air guns in order that the actual instant of firing of each gun, the shot instant, may be known and accurately controlled, for example, so that the shot instants may be synchronized, or sequenced, or otherwise controlled in accordance with a predetermined program.
In seismic surveying in a medium, such as water, acoustical energy is generated by air guns submerged in the water and is utilized to investigate subsurface geological conditions and formations. For this purpose, one or more of such air guns are submerged in the water, and compressed air, or other gas or gases under pressure, is fed to the submerged guns and temporarily stored therein. At the desired instant, the seismic source air guns are actuated, i.e., fired, and pressurized gas is abruptly released into the surrounding water. In this manner, powerful acoustic waves are generated capable of penetrating deeply into subsurface material to be reflected and refracted therein by the various strata and formations. The reflected or refracted acoustical waves are sensed and recorded to provide information and data about the geological conditions and formations.
It is also possible to submerge such seismic sources in marsh land, swamp or mud areas which are infused with sufficient water that the seismic surveying apparatus described herein can be used. Accordingly, the term "water" as used herein is intended to include marsh land, swamp or mud which contains sufficient water to enable such apparatus to be used.
It has been found advantageous in marine seismic exploration to generate a plurality of acoustic waves from an array of air guns in order to provide a composite acoustic wave of satisfactory amplitude and frequency content. Air guns having various volume capacities are generally utilized in such arrays in order to produce a composite acoustic wave having a broad frequency band, as air guns of different volumes generate acoustic waves with different frequency spectrums. For further background on air guns and solenoid valves, reference may be made to U.S. Pat. Nos. 3,249,177; 3,379,273; 3,779,335; 3,588,039; 3,653,460; 3,997,021 and 4,038,630 assigned to the same assignee as the present application.
In the past, attempts have been made to synchronize the timing of the solenoid-controlled valves which trigger the individual air guns. However, the time at which a solenoid plunger actually becomes actuated in response to an electrical signal will vary with each valve, and then there are further variables caused by the mechanical structure of each air gun, such as dimensional tolerance variation, slight differences in shuttle mass, different frictional drag of moving parts, wear of parts, and so on. When the individual air guns are not actually operating in accordance with the prescheduled precisely timed relationship, then the downgoing seismic waves are not in accordance with the predetermined survey program, and less than optimum results are obtained. For example, if the survey program calls for all of the air guns to fire in synchronism, and they are not synchronized, the downgoing seismic waves are misaligned. Misalignment may cause reduction in total amplitude of the downgoing seismic waves or cause the generation of seismic waves with undesirable frequency spectra, or both, resulting in data collection with reduced penetration and resolution.
A solution to the problem of inaccurate firing ("shot") instant as set forth in U.S. Pat. Nos. 4,034,827 and 4,047,591, assigned to Texas Instruments, Incorporated, has been to sense movement of a gas-releasing shuttle within each air gun by magnetic, electromagnetic or electrostatic field effects to establish the precise instant of firing of each individual air gun. The firing signals applied to the respective air gun solenoids are then delayed or advanced relative to each other in subsequent firings to properly synchronize or sequence the firings of the respective air guns. In those patents, the air guns are modified in order that a field such as a magnetic field be set up. The air guns are further modified in order that variations in that field caused by movement of the shuttle can be detected.
In the final paragraph of each of the above two patents, possible arrangements for producing a magnetic, electromagnetic or electrostatic field and sensing variations in that field were listed. A primary disadvantage of each of the embodiments suggested in these patents is that some one or more complex modifications of the air gun itself is required. Such modifications of an air gun to include means for generating a magnetic, electromagnetic or electrostatic field plus means to sense variations in that field are generally complicated procedures which cannot be accomplished by an operating crew and even such modification of air guns yet to be manufactured is costly. Further, any repairs by an operating crew to a sensor which is built into the air gun is time-consuming and costly.
As used herein, the term "gas" is used in a broad sense to include any compressible gaseous fluid which may be utilized in an air gun, and it includes (but is not limited to) air, steam, nitrogen, carbon dioxide, gaseous products of combustion, and mixtures thereof.
As used herein, the term "misfire" or similar wording means the same thing as "no fire" as used in said patents of Texas Instruments. This term "misfire" also includes a condition of continuous leakage of pressurized gas from an air gun. The term "malfunction" or similar wording means the same thing as "auto fire" as used in said patents.